Ultrasonic piezoelectric motors are widely known, and may conveniently be categorized into two major groups: piezoelectric motors with traveling acoustic waves and piezoelectric motors with standing acoustic waves. In the first case, the engagement of the piezoelectric actuator with the rotor is effected over a large surface, and such motors are identified as motors with surface contact. In the second category, the engagement of the piezoelectric actuator with the rotor is effected over a rectangular surface of small width, and these piezomotors are identified as motors with basically a line or point contact. The working/contact ends of such actuators move in an elliptical path in which, during part of this movement, the contact ends engage the rotor of the motor and during the rest of the movement they disengage and retract to an initial starting position. Such motors are disclosed in U.S. Pat. Nos. 4,019,073; 4,400,641; 4,453,103; 4,959,580, which are incorporated herein by reference.
U.S. Pat. No. 6,242,850 B1, also incorporated herein by reference, discloses a motor with a planar piezoelectric actuator and engagement with the rotor along a contact line. This motor comprises one or more actuators arranged around the rotor. Each actuator has at least one piezoelectric resonator, generating mechanical oscillation and with the working end directly pushing so as to rotate the rotor. Thus, the mechanical periodic oscillations of each resonator apply pressure to the rotor. The pressure applied does not exceed the natural elastic limit of the rotor, so that after each successive depression the rotor surface would restore completely to its initial state. This control of pressure applied to the rotor extends the service life of the motor, and is achieved by appropriate selection of the relative hardness characteristics of the rotor and the working end of the actuator.
Thus, in the motor of the prior art, the task of extending the motor life is achieved by improving wear resistance of the parts that are subject to applied forces and necessary friction i.e. the actuator working end and the rotor.
In such systems, issues relating to catastrophic failure, i.e. cracking of the piezoelectric actuator, are neglected. The actuator of the prior art motor illustrated in U.S. Pat. No. 6,242,850 B1 is generally made of a thin flat rectangular piezoelectric element, which acts as a piezo resonator polarized across its width. In order to ensure the necessary elliptical path for the motion of the actuator working end, frictionally engaged along the contact line with the rotor, such a motor is generally excited at a frequency close to the first longitudinal mode of mechanical oscillation across the length of the resonator. Moreover, the thickness of the resonator is chosen in such a way that a bending mode of oscillation is simultaneously excited, which is matched in frequency and phase to the longitudinal mode. This is achieved by selecting an appropriate ratio of length to thickness. In general, with an actuator 50 to 70 mm long, its thickness is in the order of several millimeters in order to satisfy the above requirement. The superposition of these two orthogonal oscillations determines the nano-elliptical motion path of the contact line (or point). During the motor operation, very often, a situation can arise when the actuator separates from the rotor for a few milliseconds (e.g. due to internal or external vibration, poor contact owing to dissimilarity between the materials of the rotor and of the actuator working end, wobble of the rotor, etc.).
For the brief duration of the separation, the actuator becomes load-free. As a result, the amplitude of the resonator oscillations increase (i.e. its quality factor “Q” increases). The resulting increased mechanical stress can cause cracking of the resonator element.
Because the maximum mechanical strain in the system is at its center (owing to the first longitudinal mode), the piezoelectric element can break in the area close to its center. The probability of such failures increases with increasing power of the motor, or its angular speed of rotation, which sharply decreases the energy efficiency of the motor and does not allow attaining the required mechanical parameters when the electric power of the motor is increased.
In prior art piezomotor design, an increase of torque is problematic because it is known that in order to increase the torque, in the first place, an increase of the pressure of the actuator against the rotor must be effected by increasing spring tension. Because the surface of the rotor of the prior art is elastic, the increased spring tension will increase the initial depression of the rotor surface, possibly leading to plastic deformation of the rotor. This would sharply decrease the system's efficiency, reduce the torque and eventually result in jamming of the motor.